Filter system



S. HEYTOW FILTER SYSTEM June 9, 1964 Filed May 27, 1960 M w mw Unitedl States Patent FOffice 3,136,838 Patented June 9, 1964 3,136,838 FILTER SYSTEM Solomon Heytow, Canoga Park, Calif., assignor, by mesne assignments, to Warwick Electronics Inc., Chicago, Ill., a corporation of Delaware Filed May 27, 1960, Ser. No. 32,183 5 Claims. (Cl. 8f4-1.11)

This invention relates to filter systems, and more particularly to a filter system for an electric musical instrument such as an electronic organ.

It is an object of this invention to provide an improved and simplified filter for insuring the substantial elimina- Output from the semi-octave bus 28 is applied to the input terminal 43 through a coupling or isolating resistor 46; output from the bus 29 is applied to the terminal 44 through a resistor 47 and so on up the filter.

It is a characteristic of the tones produced or delivered by the generators 11-26 that they are complex in character, and may depart widely from pure sine waves in harmonic composition.v The principal purpose of the filter 31 is to reject harmonics from the outputs of each generator 11-26, so that a substantially pure fiute tone may be derived for application to the voice switching of the organ.

' The complex components 'of the generators 11-26 are particularly adapted rfor an electric key board instrument such as an electronic organ. f

It is another object of this invention to provide a filter so designed that adjacent successive notes on the key board may be grouped together and applied to a common input terminal of the filter to produce, at the output of the filter, substantially pure fundamental or fiute tones.

In accordance with the foregoing objects, a preferred form of the present invention is shown in the attached drawings, wherein:

FIG. 1 is a diagram of the filter; and

FIG. 2 is a schematic diagram explaining the functioning of the invention.

Referring to FIG. 1, the numerals 11, 12, 13, 14, 16 and 17 represent successive tone generators delivering sucused in other formant circuits taken also from the generators.

To employ a separate filter for each generator 11, 12, 13, etc., which would eliminate all but the sine wave from the respective tone generator, would be extremely expensive and bulky. Past practice has been to group adjacent or successive tone generators into groups,.each spanning less than one octave and supply the common output of the group to an individual filter. If the filter had perfect cut-off characteristics, one could group 12 successive notes, representing a full octave, because even the second harmonic of the lowest tone would be just above the fundamental of the highest tone, and therefore outside the range of the filter. However, it has been found in practice cessive half tones on the chromatic music scale. Generator 11, for example, may put out the note C1, 12 puts out Clit, and so on up to 17, which generates the note F1. A complete piano type key board of notes is represented, in that succeeding generators 18,19, 21, 22, 23 and 24 generate the notes F1# through B1, respectively. This represents one complete octave on the key board. The succeeding generators` 26 continue successively up the chromatic scale. v

The octave 11-24 is divided in two, in that the first group of six generators 1117 is fed through isolating resistors 27 to a common semi-octave collecting bus 28, while the upper half of the octave 18-24 is fed to another semi-octave bus 29. Switches 30, which may be closed by depression of a key, selectively connect the tone' generators to the respective buses 28, 29.

In accordance `with the present invention, a filter 31 is provided consisting solely of non-dissipative elements. By this is meant elements which are predominately' reactive and non-resistive. The filter 31 consists of a plurality of series-connected inductors 32, 33, 34 36, to the respective junction points of which are connected corresponding capacitors 37, 3S, 39 41. The bottom terminals of the capacitors are connected to a common that this does not give sufficient factor of safety, in that filters do not have perfect, vertical cut-off slopes. Therefore it has been the practice to group notes into semioctaves (6 tones) as shownin FIG. 2. However, the

vprovision of separate filters for even each semi-octave group is also bulky and costly, and requires that when the outputs of the several filters are combined yto a common amplifier and transducer, or loud speaker, there must be coupling or isolating impedances which cause further loss of power.

It will be noted in FIG. l that the semi-octave group filtering ofthe present invention is achievedin a single filter 31 having essentially no dissipative elements, so that minimum loss is suffered between the several input terminals 43, 44 and the single output` terminal 48,'which is applied to an amplifier 49 and thence to a loud speaker 51, or other utilizing circuit.

The functioning or operation of FIG. 1 may be explained with reference to FIG. 2, wherein the action of the semi-octave 18-24 applied to theinput terminal 44,

ground .connection 42. The connection point between f throughthe resistor 47, will be discussed. The signaly One is essentially a,

applied at 44 looks into two filters. resonant filter represented by block 52 and yconsisting essentially of the elements 37, 33 and 38; and the other is a low pass filter represented schematically by the block`53 and consisting essentially of the elements 34 and 39. The resonant filter 52 is designed to peak at approximately the frequency of the lowest note in the group of tones applied to the busr29, i.e., the generator 18. The characteristic of the low pass filter 53 is thatit passes all frequencies substantially uniformly up to a value just below the frequency of the highesty tone generatorZfi., at which point it peaks slightly and then cuts off sharply just above the frequency of the tone generator 24. Thus, all frequencies above B1 are cut off. The composite or aggregate characteristics of theV two filters 52 is a generally horizontal pass curve rising rather gradually with increase in frequency, having a slight peak at F1#, dropping off slightly,

' then rising slightly at B1, and finally dropping to cut-off quite sharply. This leaves a slight saddle in the center of the response curve, but this is not serious.

The combined action of the resonant filter 52 and the low pass filter 53 serves to give the high frequency cutoff a very sharp vertical slope, which insures that even the lowest possible harmonics of the tone generators 18- 24 will not emerge from the filters 52, 53. Thus the filters 52, 53 insure that only the fundamental components of the tone generators 18-24 pass rightward through the filter system and emerge at the output terminal 48.

The present invention makes unique and elegant multiple use of the non-dissipative components. As noted, the resonant filter 52 for the input terminal 44 consists of the elements 37, 33 and 38, and the low pass filter 53 consists of the elements 34, 39. The elements 33 and 38 also constitute the low pass filter for the input terminal 43 next below the input terminal 44, and the capacitor 37 also constitutes a portion of the resonant filter for the terminal 43. The remainder of the resonant filter for terminal 43 consists of the inductor 32. Since the input terminal 43 is the lowest terminal in the filter 31, the left hand capacitor which would otherwise be necessary may be eliminated, and the left hand terminal of the inductor 32 may be directly grounded, as shown. This is because the effect of the two capacitors in the resonant filter operation is that of two series-connected capacitors, and the same effect may be achieved by eliminating one of the capacitors and cutting the value of the inductor 32 to half of what it would have otherwise been. This `leaves the resonant frequency the same value.

In a similar way, the third input terminal 54 employs the elements 38, 34 and 39 as its resonant filter, and employs additional elements to the right (not shown) as its low pass filter. These low pass filter elements in turn become part of the resonant filter circuit for the input terminal (not shown) that is next above 54.

ln the above description, the effect of the inductor 32, lying to the left of the capacitor 37, has been omitted. As far as terminal 44 is concerned, the inductor 32 and capacitor 37 comprise in effect a series resonant circuit tuned to a discretely lower frequency than that of the resonant filter 52 and connected in parallel therewith. This has the effect of appearing as a parallel connected capacitor, which slightly lowers the resonant frequency of the filter 52. At the same time it has the effect of decoupling the filter 52 from the sections of the master filter 31 which lie still further to the left (if in fact there are such sections). The remarks made immediately above concerning terminal 44 and elements 32 and 37 in series apply equally to terminal 54 and elements 33, 37 and 38 in series; and so on up the master filter 31. That is, when considering terminal 54, for example, the inductor 33 and all elements to the left of it may be ignored in a first order calculation.

As far as terminal 44 is concerned, the filter elements to the right of the capacitor 39 may be substantially discounted, because such elements represent either resonant filters which peak at frequencies discretely and markedly above the frequencies of 18-24 or are low pass filters which freely pass all the 18-24 frequencies Without substantial attenuation or peaking.

As noted hereinbefore, while theoretically each discrete filter input bus 28 could accommodate a full 12-note octave, theoretically, for a factor of safety it has been found advisable to divide the tones into semi-octave groups, thereby to insure that no overtone will get past the blocking action of the filter and come out at the output terminal 48. In this way substantially pure sine waves are delivered as flute tones to the output terminal 48.

There may also be derived from the filter 31 simultaneously with the fiute tones at 48, any number of modified complex tones. This may be done by injecting and/or removing signals or tones at any desired terminals 43, 44, 54. Such tones are then employed in other stops or formant circuits.

An excellent circuit embodying this invention has been constructed employing a value of 0.068 microfarad for cach capacitor 37, 38, 39, 41. Each section of the filter is tuned one-half octave higher than the one below. Therefore the value of the inductance is halved with each step upward. Inductor 33 is 64 henries, 32 is 32 henries, the next is 16 henries and so on up. Inductor 32 would be 128 henries if there were a capacitor at its left terminal, but since it is grounded, and has only 37 in its resonant circuit, its value is halved, to 64 henries, giving the same resonant frequency. The input impedance of amplifier 49 is 10,000-20,000 ohms, and each input resistor 46, 47, etc. is about 100,000 ohms.

While the instant invention has been shown and described herein in what vis conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom Within the scope of the invention which is therefore not to be limited to the details disclosed herein but is to be afforded the full scope of the claims.

What is claimed is:

1. Filter system comprising aplurality of series connected, reactive elements, a plurality of input terminals connected respectively' to the junctions of said reactive elements, a single output terminal reactive circuit means connecting said output terminal to said reactive elements, a plurality of successive signal generating sources each effective to deliver a signal of complex wave shape, said sources generating signals having fundamentals of successively increasing frequency, and circuit means connecting said successive signal generating sources selectively to corresponding ones of said input terminals.

2. System in accordance with claim 1 wherein the fundamental frequencies of each group of successive, adjacent, signal generating sources span less than one octave.

3. Filter system comprising a plurality of input terminals, inductive elements connected between adjacent said input terminals, a single output terminal, an inductive element connected between the last succeeding input terminal and said output terminal, a common input-output terminal, a plurality of capacitive elements corresponding to said input terminals and connected between the respective input terminals and said common input-output terminal, a plurality of successive signal generating sources each effective to deliver a signal of complex wave shape, said sources generating signals having fundamentals of successively increasing frequency, and circuit means connecting said successive signal generating sources selectively to corresponding ones of said input terminals.

4. Filter system comprising a plurality of tone producing means of successively increasing fundamental frequencies, said tone producing means being grouped into series of tones, each group spanning less than one octave, circuit means interconnecting the outputs from the tone producing means of each given group and to a common output terminal for said given group, a filter comprising a plurality of series-connected, inductors, circuit means connecting the several outputs of said groups to respective interconnection points between said inductors, one electrical end of said series-connected inductors constituting an output terminal of the filter, a plurality of capacitors each having one terminal connected to a respective one of said interconnection points, the other terminals of said capacitors being connected together, and another output terminal for said filter connected to said last named terminals of said capacitors.

5. Filter system comprising a plurality of interconnected filter elements, a plurality of input terminals connected at different points to said interconnected elements, a plurality of successive signal generating sources each effective to deliver frequencies, the fundamentals of which span less than an octave, means connecting the outputs of said several signal generating sources to the respective said input terminals, said elements constituting, for each of said input terminals, a resonant lter section peaked approximately at the lowest fundamental frequency applied to the respective terminals, and a 10W pass lter rsection cutting of just above the highest fundamental frequency applied to the respective terminal, elements of the ilter sections for any given terminal also constituting elements of the lter sections for the terminals immediately above and below said given terminal.

References Cited in the le of this patent UNITED STATES PATENTS Blumlein et al Nov. 18, 1941 Anderson Feb. 14, 1961 FOREIGN PATENTS i Germany Feb. 6,' 1958 

1. FILTER SYSTEM COMPRISING A PLURALITY OF SERIES CONNECTED, REACTIVE ELEMENTS, A PLURALITY OF INPUT TERMINALS CONNECTED RESPECTIVELY TO THE JUNCTIONS OF SAID REACTIVE ELEMENTS, A SINGLE OUTPUT TERMINAL REACTIVE CIRCUIT MEANS CONNECTING SAID OUTPUT TERMINAL TO SAID REACTIVE ELEMENTS, A PLURALITY OF SUCCESSIVE SIGNAL GENERATING SOURCES EACH EFFECTIVE TO DELIVER A SIGNAL OF COMPLEX WAVE SHAPE, SAID SOURCES GENERATING SIGNALS HAVING FUNDAMENTALS OF SUCCESSIVELY INCREASING FREQUENCY, AND CIRCUIT MEANS CONNECTING SAID SUCCESSIVE SIGNAL GENERATING SOURCES SELECTIVELY TO CORRESPONDING ONES OF SAID INPUT TERMINALS. 